Flexible films of various types are used in packaging. In the packaging of foods, in particular, such films require strength as well as the properties necessary to protect packaged foods from the adverse environmental effects of moisture and oxygen. Since no single film material was known to possess all the properties desired for a given packaging application, it was found desirable to provide multi-layer or multi-ply combinations of materials each of which possessed one or more of the desired properties. For example, polyvinylidene chloride films, which provide desired moisture and oxygen impermeability and heat sealability, can be anchored to polypropylene films which possess desired strength characteristics.
Although polyvinylidene chloride barrier films can be adhered to various thermoplastic polymeric substrates such as polypropylene films, it has been observed that the adhesion displayed between such films is not always adequate. It has therefore been proposed to use a primer in the form of a polymeric layer between the polyvinylidene chloride barrier, which functions as a top-coat, and the thermoplastic polymeric substrate to improve the adhesion between top-coat and substrate.
Solvent-based primers such as solutions of polyurethane resins dissolved in organic solvents were originally used for this purpose, as described in U.S. Pat. No. 3,023,126. While such materials were satisfactory for improving the anchoring of a polyvinylidene chloride top-coat to polypropylene substrates, existing environmental considerations now limit the use of organic solvents in packaging materials, particularly in materials for the packaging of food products.
Water-based epoxy primers, described in U.S. Pat. No. 4,214,039, do not adversely affect the environment. However, since the use of epoxy primers requires the mixing of two components immediately prior to use and since epoxy primers have a pot life of at most a few days, such primers are too cumbersome to achieve wide application.
A primer comprising an acrylic emulsion cross-linked with a water soluble melamine resin, as described in U.S. Pat. No. 4,794,136, has been found to have desirable properties. However, the need to include a separate crosslinking resin in the primer formulation is disadvantageous.
It would be desirable, therefore, to provide a flexible packaging film for use in packaging a wide variety of products, especially food products, which would utilize a self-crosslinking primer in the form of a single component, water-based System.